INFERENCE AND EVIDENCE IN COMPUTER MODELING: THREE STREAMS AND PLEISTOCENE EVOLUTION OF THEIR LONGITUDINAL PROFILES ALONG THE EASTERN MARGIN OF THE COLORADO PLATEAU IN WESTERN COLORADO

Finite-difference modeling previously completed for three streams along the eastern margin of the Colorado Plateau has provided insight regarding the effects of spatial and temporal processes along the longitudinal profiles of these streams. Modeled consequences of tectonic distortion and interaction of the stream with lithologic units of various erosional resistances correlate very well with field evidence. Modeled indications for the undisturbed Animas River clearly show that erosion rate is sensitive to climatic forcing yet may differ at nearby locations due to interaction of the stream with lithologic units of varying resistance. Terrace treads along the Animas clearly indicate climatically forced temporal variations in flow and accommodation of the stream to large variations in bedload. Indications for the Dolores River demonstrate erosional response is absorbed on the longitudinal profile and accommodation to climatic forcing may not be as evident. Inferred are less pronounced temporal variations in flow than for the Animas. Modeling renewed uplift and the abandonment of Unaweep Canyon allowed uplift rate and timing to be determined for the Uncompahgre Plateau. Clearly shown are the effects of flow deprivation to the Dolores following abandonment. Inferred are increased erosion along the Colorado from capture of the Gunnison River, and the resulting post-abandonment expression of elevation due to rejuvenated drainage development along the north and eastern margin of the Uncompahgre. Regional extension associated with opening of the no-slab window allowed isostatic adjustment and consequent renewed differential uplift to sweep abruptly northeastward across the Uncompahgre uplift beginning ~2 Ma to the south, and evident by 1.8 Ma northward in the vicinity of Unaweep Canyon. Along the crest of the Uncompahgre at Unaweep Canyon about 720 m of total renewed uplift is indicated with about 310 m occurring after abandonment at 775 ka.